1. Field of Art
The disclosure relates generally to hydraulic jars for fishing and drilling applications, including those for recovery of oil and gas. More particularly, the disclosure relates to a mechanism disposed within a hydraulic jar to provide relief of fluid pressure within the hydraulic jar and prevent the application of excessive pressure to the hydraulic jar.
2. Background of Related Art
A hydraulic jar is a mechanical tool employed in downhole applications to dislodge drilling or production equipment that has become stuck within a wellbore. Typically the hydraulic jar is positioned in the drill string as part of the bottom hole assembly (BHA) and remains in place throughout the normal course of drilling the wellbore. FIG. 1 is a simplified schematic representation of a conventional hydraulic jar. The hydraulic jar 100 includes an inner mandrel 105 slidingly disposed within an outer housing 110 with a central flowbore 180 therethrough. During normal drilling operations, fluid, e.g., drilling mud, is delivered through central flowbore 180 to the drill bit (not shown). The upper end 115 of mandrel 105 is coupled to the drill pipe (not shown), while the lower end 135 of mandrel 105 is slidingly received within outer housing 110. The lower end 130 of outer housing 110 is coupled to the remaining components of the BHA (not shown). A sealed, annular chamber 150 containing hydraulic fluid is disposed between mandrel 105 and outer housing 110. A flow restrictor 155 is disposed within chamber 150 and coupled to mandrel 105, separating chamber 150 into an upper chamber 160 and a lower chamber 165. A hammer 120 is coupled to mandrel 105 between shoulders 125, 145 of outer housing 110.
When a portion of the drill string becomes stuck within the wellbore, either a tension or compression load is applied to the drill string, and the hydraulic jar 100 is then fired to deliver an impact blow intended to dislodge the stuck portion or component. For example, when a component becomes stuck below hydraulic jar 100, a tension load may be applied to the drill string, causing the drill string and mandrel 105 of hydraulic jar 100 to be lifted relative to outer housing 110 of hydraulic jar 100 and the remainder of the BHA, which remains fixed. As mandrel 105, with restrictor 155 coupled thereto, translates upward, fluid pressure in upper chamber 160 increases, and hydraulic fluid begins to slowly flow from upper chamber 160 through restrictor 155 to lower chamber 165. The increased fluid pressure of upper chamber 160 provides resistance to the applied tension load, causing the drill string to stretch and store energy, similar to a stretched rubberband. When a predetermined tension load is reached, hydraulic jar 100 is fired to deliver an impact blow. This is accomplished by releasing the tension load being applied to the drill string and allowing the stored energy of the stretched drill string to accelerate mandrel 105 rapidly upward within outer housing 110 until hammer 120 of mandrel 105 impacts shoulder 125 of outer housing 110. The momentum of this impact is transferred through outer housing 110 and other components of the BHA to dislodge the stuck component.
Alternatively, a compression load may be applied to the drill string, causing the drill string and mandrel 105 of hydraulic jar 100 to be translated downward within outer housing 110 of hydraulic jar 100 and the remainder of the BHA, which remains fixed. As mandrel 105, with restrictor 155 coupled thereto, translates downward, fluid pressure in lower chamber 165 increases, and hydraulic fluid begins to slowly flow from lower chamber 165 through restrictor 155 to upper chamber 160. The increased fluid pressure of lower chamber 165 provides resistance to the applied compression load, causing the drill string to compress and store energy, similar to a compressed spring. When a predetermined compression load is reached, hydraulic jar 100 is fired to deliver an impact blow. This is accomplished by releasing the compression load being applied to the drill string and allowing the stored energy of the stretched drill string to accelerate mandrel 105 rapidly downward within outer housing 110 until hammer 120 of mandrel 105 impacts shoulder 145 of outer housing 110. The momentum of this impact is transferred through outer housing 110 and other components of the BHA to dislodge the stuck component.
As described, hydraulic jars may be bi-directional, meaning they are capable of delivering an impact blow in both the uphole and downhole directions. Alternatively, a hydraulic jar may be uni-directional, meaning it is designed for and is capable of delivering an impact blow in either the uphole or downhole direction, but not both. Regardless, the common feature of each is that stored energy, created by stretching or compressing the drill string, is used to accelerate the mandrel of the hydraulic jar to deliver an impact blow to the outer housing. Moreover, the higher the load applied to the mandrel, the faster the acceleration of the mandrel and the greater the impact force delivered to the outer housing.
However, increased tension or compression load to the hydraulic jar may come at significant cost. Due to structural limitations of the hydraulic jar, excessive hydraulic fluid pressure may cause failure of seals within the hydraulic jar and/or the body of the hydraulic jar itself, i.e., the mandrel or the outer housing. Failure of the hydraulic jar results in loss of the tool itself, the inability to dislodge equipment stuck within the wellbore, and increased drilling time and expense. Given the costs associated with failure of a hydraulic jar, these tools are typically operated at only a fraction of their capacity. For example, the hydraulic jar may be fired when the tension or compression load applied reaches only three-fourths of the structural capacity of the hydraulic jar, rather than nearer the capacity of the tool. Due to frictional losses, the load delivered to the downhole end of the drill string will be less than the applied tension or compression load. Even so, the applied load is not typically increased to compensate for frictional losses because to do so increases the risk of jar failure. Hence, as a result of operating the hydraulic jar at a fraction of its capacity and frictional losses, the impact blow delivered by the hydraulic jar may be insufficient to dislodge stuck equipment or additional impact blows may be required, both increasing the time and cost associated for drilling the wellbore.
Accordingly, there remains a need for a hydraulic jar that may be operated near or at its structural capacity without causing damage to or failure of the hydraulic jar as may be caused by excessive hydraulic fluid pressure within the hydraulic jar.